1. Field of the Invention
The present invention relates generally to the production of electric power by use of geothermal water or brine and more particularly to processes for regenerating media filters used to filter the geothermal water or brine prior to reinjection thereof.
2. Discussion of the Prior Art
Large subterranean aquifers of naturally produced (geothermal) steam or hot aqueous liquids, specifically water or brine, are found throughout the world. These aquifers, which often have vast amounts of energy potential, are most commonly found where the earth's near-surface thermal gradient is abnormally high, as evidenced by unusually great volcanic, fumarole or geyser activity. Thus, as an example, geothermal aquifers are fairly common along the rim of the Pacific Ocean, long known for its volcanic activity.
Geothermal steam or water has, in some regions of the world, been used for centuries for therapeutic treatment of physical infirmities and diseases. In other regions, such geothermal fluids have long been used to heat dwellings and in industrial processes. Although efforts to further develop geothermal resources for these site-restrictive uses continue, considerable recent research and development has, instead, been directed to exploitation of geothermal resources for production of electrical power which can be conducted, often over existing power grids, for long distances from the geothermal sources. In particular, recent steep increases in the cost of pertroleum products used for conventional production of electric power, as well as actual or threatened petroleum fuel shortages or embargos have intensified the interest in use of geothermal fluids as an alternative and generally self-renewing source of power plant "fuel".
General processes by which geothermal fluids can be used to generate electric power are known and have been known for some time. As an example, geothermal steam, after removal of particulate matter and polluting gases such as hydrogen sulfide and ammonia, can be used in the manner of boiler-generated steam to operate steam turbine generators.
Naturally pressurized geothermal brine or water having a temperature of over about 400.degree. F. can be flashed to a reduced pressure to convert some of the brine or water to steam. The steam produced in this manner can then be used to drive steam turbine generators. The flashed geothermal liquid and the steam condensate obtained from power generation can typically be reinjected to replenish the aquifer and prevent ground subsidence. Cooler geothermal brine or water can often be used to advantage in binary systems in which a low-boiling point, secondary liquid is vaporized by the hot geothermal liquid, the vapor produced being used to operate gas turbine generators.
As might be expected, use of geothermal steam is preferred over use of geothermal water or brine for generating electric power because the steam can be used more directly, easily and cheaply. Consequently, where readily and abundantly available, geothermal steam has been used for a number of years to generate commercially important amounts of electric power at favorable costs. For example, by the late 1970's geothermal steam at The Geysers in Northern California was generating about two percent of all of California's electricity consumption.
While energy production facilities at important geothermal steam sources, such as at The Geysers, are still being expanded, when not already at capacity, the known number of important geothermal steam aquifers is small compared to those of geothermal brine or water. Current estimates are, in fact, that good geothermal brine or water sources are about five times more prevalent than are good sources of geothermal steam. The potential for generating electric power is, therefore, much greater for geothermal brine and water than it is for geothermal steam. As a result, considerable current geothermal research is understandably directed towards the development of economical geothermal brine and water electric generating plants, much of this effort being expended towards use of vast geothermal brine resources in the Imperial Valley of southern California.
Although, as above mentioned, general processes are known for using geothermal brine or water for production of electric power, difficult problems, especially with the use of highly saline geothermal brine, have often been encountered in practice. These problems have frequently been so great as to impede the progress of geothermal brine power plant development in many areas.
These severe problems relate primarily to the typically complex composition of geothermal brines. At natural aquifer temperatures in excess of about 400.degree. F. and pressures in the typical range of 400 to 500 psig, the brine leaches large amounts of salts, minerals and elements from the aquifer formation. Thus, although brine composition may vary from aquifer to aquifer, wellhead brine typically contains very high levels of dissolved silica as well as substantial levels of dissolved heavy metals such as lead, zinc, copper, iron and cadmium. In addition many other impurities, particulate matter and dissolved gases are present in most geothermal brines.
As natural brine pressure and temperature are substantially reduced in power plant steam conversion (flashing) stages, silica saturation levels in the brine are typically exceeded and silica precipitates from the brine, as a tough scale, onto surrounding equipment walls and in reinjection wells, often at a rate of several inches in thickness per month. Scale, so formed, typically comprises iron-rich silicates, containing varying amounts of brine impurities, and is usually very difficult, costly and time consuming to remove from equipment. Because of the fast scale forming rates, extensive facility down time for descaling operations may commonly be required at some geothermal brine facilities. Injection wells may also require frequent and extensive rework and new injection wells may, from time to time, have to be drilled at great cost.
Considerable effort has, as a consequence, been directed towards developing processes for eliminating or substantially reducing silica scaling in flashed brine handling equipment and injection wells.
To this end, a scale reduction process of particular interest causes controlled, induced silica precipitation from the brine in the flashing stage by utilizing seed crystallization techniques. By such process, when silica saturation levels in the brine are reached as a result of the brine being flashed to a reduced pressure, the "excess" silica is induced to precipitate or crystallize onto seed crystals intentionally introduced into the flashing vessels. Circulation of flashed brine and seed material in the flashing vessels enhances and accelerates the silica crystallization process. Downstream of the flashing-crystallizing stage, most of the silicious precipitate is separated from the brine in a reactor-clarifier stage. Some of the silicious precipitate from the reactor-clarifier stage is pumped back upstream to the flashing-crystallizing stage as seed crystals. Clarified brine is discharged from the reactor-clarifier stage for return to the ground by the injection stage.
Although most of the silicious precipitate is separated from the brine in the reactor-clarifier stage, significant amounts of precipitate are nevertheless carried along in suspension with the discharged brine. Typically, suspended material is in the form of very small particles less than about 6 microns in size which are not readily separated from the brine. Concentrations of these suspended particles in the brine discharged from the reactor-clarifier may be in the approximate range of 100 to 300 parts per million (ppm) by weight.
At very high brine flow rates, which may, for example, be about 1.3 million pounds per hour for a 10 megawatt geothermal brine power plant, these fine, suspended silicious particles cause scale buildup in downstream equipment and gradually plug up the brine injection wells and fissures in the formation into which the brine is injected. To prevent such scaling and plugging problems, brine discharged from the reactor-clarifier stage is typically flowed through a filtering stage constructed to remove most of the fine suspended particles from the brine. When properly operating, the filtering stage, which may comprise several filters in series and/or parallel relationship, ordinarily removes suspended particles in the brine larger than about 2 micron in size and typically reduces the particle concentration in the brine discharged from the filters to less than about 30 ppm and often to less than about 10 ppm. Such concentrations of 2 micron or less particles in the brine effluent can generally be tolerated by the brine reinjection stage without excessive scale formation and/or injection well plugging.
The fine silicious particles removed from the brine by the filters, of course, accumulate in the filters and it has been found that, in time, the particles aggregate into large masses which may reach several inches across. Agglomerations of such nature cause loss of filter effectiveness and eventual choking off of brine flow through the filtering stage.
Media filters used for brine filtering are commonly cleaned by backwashing the filters with a flow of filtered brine. However, most types of filters must be out of service during backwashing; it is thus ordinarily desirable to backwash no oftener than is necessary to maintain proper filtering operation. Another reason for not backwashing the filters any more frequently than is necessary is that backwash water must be provided and dirty backwash water must be disposed of. On the other hand, if backwashing is too infrequent, the silicious agglomerate may be impossible to remove and repacking of the filters is then necessary at relatively great expense.
Because of cohesiveness of the silicious particles removed by the media filters, even relatively small agglomerations are difficult to remove by backwashing; hence, relatively frequent backwashing has been found necessary in order to minimize silica agglomeration. For example, backwash intervals as short as 8 hours have, in some cases, been found necessary.
It is generally known, as evidenced by the disclosures of U.S. Pat. Nos. 3,680,701 and 4,191,652 to Holca and Whitmore, respectively, that backwash effectiveness of some types of media filters not used for geothermal brine, may be greatly enhanced by a pre-backwash, pressurized air "scour" of the filters. Air, under pressure, flowed through the filters in the backwash direction agitates the filter media and tends to break up and/or loosen filtered-out material so that the subsequent water backwash can effectively flush out entrapped particles.
However, such air scouring is inappropriate for media filters used for filtering silica-rich geothermal brine, since enhanced corrosion and scaling are known to be caused by contacting geothermal brine with air. Silica precipitation from geothermal brine has been found to be enhanced by the presence of ferric ions, the resulting precipitate being generally of an iron-rich silicious material, the exact composition of which is quite complex and is, therefore, considered not entirely understood. Silica-rich geothermal brine normally contains an appreciable amount of dissolved iron, mostly, however, in the ferrous ion form which does not induce or enhance silica precipitation. If the brine is contacted with air, some of the ferrous ions are oxidized to ferric ions, which do greatly enhance corrosion and scaling. As a consequence, great care is typically taken to avoid or minimize contacting geothermal brine with air. It is therefore considered that geothermal brine media filters should not be air scoured in order to avoid scaling and corrosion in the filters and downstream equipment. Nevertheless, improved media filter regenerating processes are required to increase the time between backflushes and/or improve backflush effectiveness, thereby reducing power plant operating costs.
It is, therefore, an object of the present invention to provide an improved process for regenerating geothermal brine media filters, which includes non-air, pre-backwash scouring of the filters.
Another object of the present invention is to provide an improved process for regenerating geothermal brine media filters which includes using steam obtained from the geothermal brine for scouring the filters.
Additional objects, advantages and features of the present invention will become apparent to those skilled in the art from the following description, when taken in conjunction with the accompanying drawing.